liblzma: Update lzma_lzip_decoder() docs about trailing data

liblzma: Update lzma_lzip_decoder() docs about trailing data

Don't say that the .lz format allows trailing data. According to the
lzip 1.25 manual, trailing data isn't part of the file format at all.
However, tools are still expected to behave as usefully as possible
when there is trailing data.

Fix the description of lzip >= 1.20 behavior when some of the first
bytes of trailing data match the magic bytes. While the lzip 1.25 manual
recommends that none of the first four bytes in trailing data should
match the magic bytes, the default behavior of lzip 1.25 treats
trailing data as a corrupt member header only if two or three bytes
match the magic bytes; one matching byte isn't enough.

Reported-by: Antonio Diaz Diaz
Link: https://www.mail-archive.com/xz-devel@tukaani.org/msg00702.html